The invention relates generally to a wet press of a paper making machine and more particularly to a wet press having a tiltable felt-tightening roll.
Wet presses of paper-making machines having the following elements for treating paper webs are known: a pair of first rolls defining a roll or press gap through which the paper web is conducted, second rolls defining a closed loop path which includes the roll gap, a belt-shaped felt guided by and revolving around said second rolls in the closed loop path with the web and felt being simultaneously conducted through the roll gap, the second rolls including a tightening roll pivotally supported at one end with the felt being looped around the tightening roll with a section of the felt running toward the tightening roll and a section of the felt running away from the tightening roll and a manually operable means for tilting the tightening roll by a given tilt angle which lies in a plane located in the space between the felt sections and which passes through the longitudinal axis of the tightening roll.
In such wet presses, as the felt and paper web to be drained are simultaneously conducted through the roll gap, water is pressed out of the paper web and is transferred onto the felt web. The absorbed water is removed from the felt at another point along its closed loop path by, for instance, a suction roll.
The pair of rolls forming the roll gap, along with their guides which engage roll journals and the elastically resilient felt, form a vibrating system with a large number of resonance vibrations that can be excited during operation of the wet press. Vibrations in the range of about 20 to 150 Hz typically occur and manifest themselves as a loud humming sound.
Such phenomena becomes particularly pronounced if at least one of the rolls of the pair is a flexure-controlled roll, i.e., a hydraulically supported rotatable hollow roll having a stationary crosspiece extending through the hollow roll and forming a small clearance space therewith which contains the supporting liquid. The vibration problem is exacerbated with flexure-controlled rolls because such rolls comprise several parts which add to the number of vibratory degrees of freedom.
The vibrations and noise produced are not only a nuisance, but also have a negative effect on the operation of the wet press. This problem applies particularly to so-called "beaten path vibrations" which either occur in the plane connecting the axes of the rolls forming the roll gap or have a component in this plane. When the rolls vibrate toward each other in this plane, the portion of felt located in the roll gap at the instant the vibration occurs is greatly compressed and forms a mark in the felt. During the remaining times when the rolls vibrate away from each other little or no compression results. Under certain geometric conditions, after some period of operation the marks develop in the felt in a line pattern which extends transverse to the direction of felt motion and becomes permanently impressed on the felt. These marks can excite resonate frequencies of the wet press thereby causing amplification of the vibrations.
The felt compression along the lines also hampers the absorption characteristics of the felt. The felt has a reduced service life when vibrations occur because its drainage action is reduced due to the line pattern which results in linear hardened zones. Therefore, the felt must be replaced sooner than if it was uniformly stressed under diminished vibratory conditions. The generation of the vibratory marks in the transverse line pattern greatly reduces the service life of the felt which would be otherwise possible. Furthermore, these irregularities in the absorption capability of the felt adversely affect the treated paper web as uneven draining of the paper web results therefrom. Additionally, the transverse line pattern remains Visible after the paper is completely dry. Hence, these vibrations lead to diminished paper quality.
The foregoing problems indicate that there is great interest in preventing the occurrence of the self-amplifying vibrations in a wet press generated by the transverse line pattern in the felt. In known wet presses of the prior art means are provided to attenuate the excitation of vibrations by transverse marks in the felt web. Typically this is accomplished by tightening the felt by means of a pivotally supported tightening roll. Tilting of the tightening roll tightens the felt into a generally parallelogram-like shape which prevents the vibratory marks from forming in a transverse direction, i.e., perpendicular to the running direction of the felt. The marks are now formed at a certain angle and the entire length of a mark does not reach the roll gap simultaneously. This largely cancels the excitation effect of the marks arriving at the roll gap.
If the tightening roll is tilted and the felt is not lengthened on its other side, destruction of the felt and in particular, lateral run-off of the felt would occur due to the shear stress induced. Therefore, the effects of the displacement or tilting of the tightening roll at one edge must be compensated by corresponding displacement at the other edge or by other similar measures.
In the known wet presses discussed above, the tightening roll is tilted manually by a spindle. The machine operator recognizes, from experience, by the noise generated by the wet press when tilting intervention is necessary. He then operates the spindle to tilt the tightening roll by a certain amount until the noise is reduced. After several hours or days, the felt has run-in in its new tightened position and new marks begin to form which excite the corresponding resonance vibrations of the roll arrangement and lead to amplified vibrations At such time, more intervention is required.
One of the problems With the prior art arrangement is that continuous monitoring and correspondingly great experience of the machine operator are required in order to maintain efficient, orderly and vibration-free operation of the wet press. The present invention solves this problem by providing for operation of the vibration suppression means in the known wet press independent of the continuous attention and intervention of the machine operator.